A method of preparing an ultra-nanoscale-LC monolithic separation medium for use in capillary columns, or channels in microfabricated devices (microchips), and capillaries prepared by the method are disclosed. The application of moderate positive pressure to both ends of the capillary during the monolith polymerization process permits the preparation of monolithic capillary columns having very low i.d., e.g., 25 μm and smaller, with enhanced mass transfer properties and low back pressures, and excellent column-to-column reproducibility of retention times.